Process for the production of steel



Aug. 20, l

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P, 5 PERCENT FIG2.

957 o. cUscoLEcA ETAL 2,803,534

PROCESS FOR THE PRODUCTION OF STEEL Filed Dec. 22, 1954 C i. 5 zo L) a u' LO E` "r s'. 7 Mn 2 4- 6 8 O I2 I4- IG I6 2O 22 23 MINUTES IOO LIJ S E 8O z a @o u! D.

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72 mvmms Xiu/R ATTORNEYJ United States Patent O PROCESS FOR THE PRODUCTION OF STEEL Otwin Cuscoleca, Velden/ Werther See, Felix Grohs, Leon ben, Wolfgang Khnelt, Judenburg, and Kurt Rosner, Leoben/Donawitz, Austria, assignors to Oesterreichisch-Alpine Moutangesellschaft, Vienna, Austria Application December 22, 1954, Serial N o. 476,872

Claims priority, application Austria August 7, 1954 6 Claims. (Cl. 75-60) The invention relates to the production of steel and particularly to the production of high grade steels by refining carbon-containing iron baths with oxygen enriched gases.

Prior processes for producing steel by refining carboncontaining iron baths can be divided into two general groups. One group comprising the electric and the open hearth processes employs solid refining means i. e. solid substances rich in oxygen and particularly rich in iron oxides. The action of such refining means, added in the solid state, is partly responsive to endothermic reactions and a relatively long time is required for obtaining the results desired and consequently heating from outside is indispensable. This disadvantage is offset at least to a certain extent by the fact that high grade steels are produced.

The other group of processes for producing steel by refining carbon-containing iron baths is characterized by the use of gaseous oxygen as refining means. Usually these refining processes will operate without the introduction of heat from outside as the heat needed in the process is produced by the exothermic reaction between the iron bath and the refining gases. The most widely used processes of this group are the basic Bessemer process (Thomas process) and the acid Bessemer process wherein air as refining means is blown from nozzles through the iron bath from below. These processes are economically superior to the processes applying solid refining means, chiefly with regard to the capital costs, the speed of production and the possibility to operate without any heating from outside. inasmuch as the acid process does not remove phosphorous and sulfur from the pig iron when refining it into steel, and as the pig irons available nearly always contain more phosphorous than is permissible in the finished steel, in most cases the basic Bessemer process (Thomas process) is employed, although the latter suffers from the disadvantage that the pig iron employed in that process must be high in phosphorous to assure the heat balance required.

Although the Thomas process has certain economical advantages, it should not be overlooked that open hearth steel is of higher quality than the Thomas steel. At first, the higher quality was attributed to the lower phosphorous and nitrogen content of the steel so that many attempts have been made to modify the Thomas process to reduce the phosphorous and the nitrogen content of the Thomas steel. However, experience has shown that the improved Thomas steel, even if it has the same phosphorous and nitrogen content as the standard open hearth steel, is not equal to the latter for many applications. Recently it has been found that this inferiority of the improved" Thomas steel with regard to the open hearth steel is due to the higher oxygen content of the former.

Other processes have been suggested for refining carbon-containing iron baths by means of oxygen-containing gases. Electric and the open hearth furnaces have been modified to include lances for blowing technically pure ICC oxygen on the bath from above or into the bath in order to accelerate the process and reduce the amount of heat required to be applied from outside. Other processes are characterized by being carried out in a converter or a similar vessel into which oxygen containing gases and in particular high percentage or technically pure oxygen are blown from above on or into the bath.

For example, a process has been proposed in which slag forming materials and great quantities of iron oxidecontaining substances are introduced into a bath of molten pig iron and in which a jet of air, oxygen enriched air, or pure oxygen is directed from above onto the surface of the bath in such a manner that a whirling motion is produced thoroughly mixing the iron bath and the solid iron oxide-containing substances. In this operation, refining is chiefly produced by the iron-oxide containing additions, and is facilitated by the intimate mixture of iron and slag, although it is possible that direct refining takes place under the action of the gaseous oxygen to a smaller extent. This process is said to permit of the production of steel containing a low percentage of phosphorous from pig irons high in phosphorous. However, the reaction between the great quantity of solid iron oxides and the elements to be removed from the pig iron yields an unfavorable heat balance in addition to the fact that the steel contains a substantial proportion of oxygen as the result of the high FeO contents of the slag.

Other processes have been suggested making use of a converter or a similar unheated vessel in which refning is chiefly accomplished by blowing oxygen-containing gases directly or laterally from above into the bath. Bessemer previously had suggested that oxygencontaining gases might be blown into the iron bath by means of a pipe extending below the surface of the bath.

Recently it has been proposed to blow oxygen-coning gases against the surface of the bath (e. g. by means of a nozzle operating at supersonic speed) in such a manner that the gas jet penetrates deeply into the bath. These processes are the result of the concept that it is essential to bring the oxygen of the blast into the interior of the metal bath in order to obtain heats of short duration characteristic of the Bessemer process and to convert the carbon-containing iron bath into a sufficiently hot and readily castable steel, without heating the bath exteriorly. However, these prior attempts overlooked the fact that the reaction between the oxygen and the iron bath takes place within the latter so that iron oxide is formed and retained in the bath itself, the result being a steel that is always high in oxygen.

A process has also been proposed in which one or more nozzles were provided for blowing oxygen enriched gases against the surface of the bath in such a way that the slag, the iron oxide, of which a certain quantity is added, and the metal bath itself are turbulently intermixed, whereby operating temperatures of more than 2500 F. are used. The object of this process is to sharply reduce the phosphorous content of the finished steel by a suitable addition of lime and by accelerating the formation of slag. In this process, it is unimportant as to whether the oxygen-containing gas is blown onto the surface, or is blown into the metal bath beneath the surface, or is introduced slightly beneath the surface. ln order to remove the phosphorous rapidly, this process is carried out under heavily oxidizing conditions so that the steel produced will always contain a relatively high percentage of oxygen.

Finally, other processes have been suggested in which oxygen-containing gases, and preferably high percentage oxygen, are intentionally blown onto the surface of the bath thus restricting the reaction between the oxygen and the metal bath to the surface of the latter, in consequence whereof the iron-oxide formed is readily absorbed by the slag which is rapidly liquified by the maximum temperatures occurring in the reaction center on the bath surface. By processes of this kind it is possible to produce a steel of low nitrogen and phosphorous content, but the oxygen content of these steels is bound to vary and is generally found to be on a higher level than in the standard open hearth steels.

In accordance with the present invention, it has been found that it is possible to produce in a converter or any other suitable unheated vessel a high grade steel not only having low phosphorous and nitrogen content, but having at the same time an extremely low oxygen content comparable to that of a good open hearth steel. This result is attained by directing high percentage oxygen from one or more nozzles arranged above the bath onto the surface of a carbon-containing iron bath in a restricted area to produce a limited reaction center having a maximum temperature on the surface of the bath while regulating the supply of oxygen in such a manner that the consumption of oxygen per ton of metal charged does not greatly exceed the theoretical quantity of oxygen necessary for this process per ton of the metal charged.

It has been ascertained that a high grade steel having a low percentage of oxygen, corresponding to that contained in good open hearth steels, is obtained if the oxygen consumption is as small as possible and does not exceed, in any case, 112% of the theoretical quantity for conversion of the iron to steel. In general, the consumption of oxygen should be about 105% of the theoretical or slightly below that limit.

More particularly, in accordance with the invention, a process is provided for making low-oxygen steel out of carbon-containing iron baths by blowing oxygen enriched gases, and preferably high percentage oxygen, from above onto the surface of the bath, to which slag forming, and particularly lime-containing additions, iron oxide containing substances, and fluxes have been added. The refining operation is conducted Without heating the receptacle for the bath.

In order to determine the amount of oxygen theoretically required for producing steel, a number of factors must be considered. In refining carbon-containing iron baths, oxvgen is required for oxidizing the elements accompanying the iron down to the percentages of these elements which must remain in the finished steel. Carbon is burnt to gaseous CO and then partly oxidized to CO2. The gas analyses conducted in connection with the process under consideration show that about 90% of the carbon is turned into CO in the converter and about 10% into CO2. This is, therefore, the ratio on which the calculations are based. These gases escape from the bath and leave the converter through its mouth. The Mn contained in the bath is turned into MnO and absorbed by the slag. Similarly, Si is oxidized into SiOz and P into P205 which also enter the slag. A part of the S is directly burnt to SO2 owing to the high temperature developed in the range of reaction between the oxygen and the rnetal bath, said SO2 escaping together with the waste gases, whereas the remainder turned into CaS, is absorbed by the slag. However, for ascertaining the quantity of oxygen theoretically required for this process, it is assumed that all of the sulfur leaving the bath is converted to SO2.

Furthermore, the high temperatures prevailing in the` zone of reaction between the oxygen and the surface of the bath cause not fully understood reactions producing an oxidizing gasification of the iron and the formation of smoke. Experience has taught that about 10 kg. of dust are produced per ton of the metal charged, the dust containing about 63% of iron, and the oxidation in the converter reaching the stage of FeO. Accordingly, in determining the theoretical quantity of the oxygen, per

ton of metal charged, it is necessary to include the amount of oxygen required for oxidizing 6.3 kg. of iron into FeO, the iron being gasied and escaping with the waste gases.

The theoretical quantity of oxygen required for carrying out the process is, therefore, equal to the sum of the quantities of oxygen necessary for oxidizing the elements, accompanying the iron, i. e. as a rule: carbon, Mn, Si, P and S, down to the percentages which are to remain in the finished steel, adding thereto the oxygen required for the formation of the dust consisting of FeO.

The quantity of oxygen actually introduced into the process is designated as oxygen consumed and comprises the oxygen of the iron oxides contained in the solid additions, as well as the oxygen in the oxygen-con taining gases blown onto the bath. There is no difliculty in analytically ascertaining the oxygen required per ton of metal charged, taking into account the quantity and the composition of the iron oxide-containing additions and the content of oxygen of the added oxygen-containing refining gases.

Oxygen-enriched gases referred to herein are gases containing more than 21% oxygen by volume.

The invention can be practiced in several different ways. A preferred method consists in maintaining the introduction of the oxygen per unit of time at a higher level during the first stage of the process than that theoretically corresponding to the oxygen requirements per unit of time necessary in said period for oxidizing the carbon and maintaining the quantity of oxygen fed per unit of time at a level lower during the later stages of the process than that which is theoretically necessary. In this method, the introduction of oxygen may advantageously be switched from an excess to a shortage at the moment, or shortly after the moment at which the carbon combustion attains its maximum speed.

A high grade steel having a low oxygen content conformable to a first class open hearth steel, can be ob tained if in the period of very rapid carbon combustion at the very high temperature caused by the exothermic reaction between the oxygen and the elements (C, S, Si, P, etc.) accompanying the iron, the introduction of oxygen per unit of time is kept at a lower, and preferably at a much lower, level than that corresponding to the oxygen quantity required in that period per unit of time for the combustion of the carbon.

This mode of operation yields very favorable conditions with regard to the oxygen consumption in its relation to the theoretical requirements in oxygen.

Another method for producing high grade, low oxygen steel involves blowing oxygen-enriched gases, and preferably high percentage oxygen on the surface of a carboncontaining iron bath in a converter or any other suitable, unheated receptacle or vessel, whereby on the surface of the bath a restricted reaction center of maximum ternperature is formed. This method consists in keeping the introduction of oxygen per unit of time during the period of rapid high carbon combustion at a lower. and preferably at a much lower, level than that of the quantity of oxygen which, at that stage, is consumed per unit of time for the combustion of carbon. This can be achieved by introducing the gaseous oxygen at a constant rate during the entire process so that less oxygen is fed per unit of time than is required during the period of high speed carbon combustion.

Moreover, it is possible to accelerate and speed up the process so as to render it more economical by introducing at first a greater quantity of gaseous oxygen per minute and then throttling the introduction of oxygen during but not before, the period of hcavy carbon combustion so that the oxygen quantity fed per unit of time lags behind the quantity of oxygen which is required for combustion of the carbon at a very high speed under the influence of the high temperatures in the converter.

This mode of operation has the advantage of allowing escasas consumption of oxygen to be reduced with the duration of the refining process remaining the same or being even reduced, so that savings can lie effected in the consumption of gaseous oxygen. A special advantage resides in that Fe can be recovered from the FeO of the slag, so as to reduce the iron losses of the refining process.

The regulation of the quantity of oxygen introduced per unit of time can be accomplished easily by controlling the rate of introduction of the oxygen into the nozzle during the blowing process. It is, however, also possible to introduce the oxygen selectively through one of two or more blowing nozzles, f. i. through blowing nozzles of different discharge cross sections which, one after the other are run into the converter together with their junetion pipes. This offers the advantage that the cross sections of the nozzles may be adapted to the respective quantity of oxygen and that by doing so the flow conditions can be kept unchanged in spite of the variation in the quantity of oxygen supplied to the converter.

During the early stages of the blowing operations, not only the accompanying elements (manganese, silicon and phosphorous) are oxidized, but also a certain amount of iron which increases the iron content of the slag. The increased iron content of the slag accelerates the formation of slag due to the rapid solution of the lime added,

which, in turn, speeds up the oxidation of the elements accompanying the iron. With an increased utilization of oxygen during the first period of the process, the temperature rises rapidly so that the period characterized by the high speed carbon combustion is reached quickly. It has been found that an increased FeO content of the bath, caused during the first period of the process by the high Fe() content of the slag, does not adversely affect the quality of the finished steel, if in the period of intensive carbon combustion, the adduction of gaseous oxygen is maintained at such a low level that the balance of oxygen necessary for the rapid combustion of carbon is taken from the oxygen dissolved in the bath and contained in the slag, whereby the iron oxide content in the slag and in the bath are likewise reduced.

The refining process may be controlled in such a manner that, in the rst operating period, as long as the combustion of the carbon is not yet going on intensively and as long as the oxidation is mainly restricted to the secondary accompanying elements Mn, Si and P in addition to certain quantities of iron, greater quantities of oxygen are fed per unit of time, whereas in the period of intensive carbon combustion the introduction of oxygen is throttled to such an extent that the gaseous oxygen fedper minute is not sufficient to cover the oxygen requirements per unit of time in view of the rapidly proceeding oxidation of carbon.

It has been found that it is advantageous to keep the oxygen introduction per unit of time below the limit theoretically necessary of oxidizing the carbon, or to reduce it below that limit as soon as vthe bath has reached a temperature between l450 C. and 1600 C. (preferably about l500 C.). Particularly favorable results have been attained if the introduction of oxygen per unit of time was kept or reduced below the oxygen requirements theoretically necessary for the oxidation of the carbon, as soon as the carbon content of the iron bath has dropped to 75-25% (preferably to about 50%) of the original carbon content. ln case of the ordinary pig iron kinds this corresponds to a carbon content of 3.0 to 1.0% (preferably about 2.0%).

It has further been found that particularly advantageous results are attained with regard to the quality of the steel, if additionally, in the period of the intensive carbon combustion, the surface pressure of the gas jet on an area whose distance from the blowing nozzle corresponds to the distance of the bath surface from the blowing nozzle, is kept below 0.75 lig/cm.2 or preferably below 0.5 kg./cm.2. At such a pressure, the jet does not penetrate appreciably into the bath or cause swirling of the bath or intensively mix the slag with the bath. As a consequence, the oxygen content of the iron bath is kept as far as possible below the oxygen balance between the bath and the slag, considering the influence of the demand by the bath for FeO to promote the combustion of the carbon.

As measuring the surface pressure of the gas jet in the converter is impossible under the conditions therein, the surface pressure is determined outside of the converter at normal room temperature in a gas medium such as air, by means of a nozzle of the same size and shape as used in the converter. The pressure of the oxygen-containing gases supplied to the nozzle can be regulated to produce the desired pressure against a surface for a selected nozzle spacing from the surface by directing the gas from the nozzle against a member of a selected area which is mounted on a sensitive balance to measure pressure.

For carrying out the process, a basic lined converter or any other suitable unheated vessel is used to receive a bath of carbon-containing liquid iron, preferably pig iron having a composition that may vary between wide limits. Before, during or after the introduction of the liquid iron, suitable quantities of scrap, slag forming materials (basic slag forming substances, preferably lime-containing substances, iron oxide containing substances and fluxes) are introduced into the converter. The scrap serves to regulate the temperature of the metal bath, its quantity being determined according to the initial temperature of the liquid carbon-containing iron which is poured into the converter, to the kind and quantity of the elements contained in the liquid iron, to be removed by oxidation in the course of the process, to the analysis desired of the finished steel and to the tapping temperature corresponding to said analysis. It is advisable to put the entire quantity of scrap into the converter before pouring in the pig iron. But if desired, the scrap may be introduced entirely or partly during the process. With regard to the slag-forming materials, preferably the pig iron is introduced into the converter first and the slag-forming substances are then dumped on the surface of the bath. However, they may also be added entirely or partly in the further course of the process. In general, CaO containing substances are used as basic slag-forming materials. It is advisable to employ incompletely burnt lime, e. g. of but Sil-% CaO, containing a minimum of impurities. The residual carbon dioxide contained in a lime of this kind is rapidly expelled at the high surface temperatures of the metal bath, whereby the pieces of lime are shattered and are thus more rapidly dissolved and absorbed into the slag. The quantity of the CaO introduced depends on the composition of the pig iron, and in particular on its P and Si contents, and should be turned in such a manner that a slag of appropriate basicity capable of a vigorous reaction is formed as soon as possible. The slag should be capable of absorbing the constituents of the pig iron removed from the bath in the course of refining and of firmly binding these constituents and especially the phosphorous.

The iron oxide containing substances are chiey iron ore, sintered iron ore or mill scale. The quantity of these iron oxide containing materials is preferably determined in such a manner that the rapid formation of an initial slag chiefly consisting of calcium ferrite is assured, so as to warrant a rapid removal of the phosphorous. The addition of fluxes depends on the composition of the pig iron and of the slag-forming substances. For instance, it may be advantageous, if the Si contents of the pig iron and the SiO2 contents of the slag-forming materials are small, to add SiOz containing uxes, such as quartz sand, crushed silica bricks, or the like. The fluxes are added for rapidly bringing about the formation of the first liquid slag which, under the influence of the high temperatures of reaction, will rapidly dissolve the lime added, and will soon reach the state of high reactivity, particularly efficient for the removal of the phosphorous. As soon as possible after the introduction of the pig iron, oxygenenriched gases, preferably high percentage oxygen, are blown onto the surface of the bath employing one or more nozzles, depending on the size of the metal charge. The nozzles, in general, are arranged vertically in respect to the bath.

The quantity of O2 blown against the surface of the charge per unit of time and per ton of metal charged depends first of all on the refining speed desired. It may be advantageous to direct, as mentioned above, the gas jet onto the bath with such a pressure that it does not penetrate into the bath, but forms on the surface of the bath a reaction center of peak temperature as in this case the area of the highest tempcraurc develops in the immediate vicinity of the slag, thus assuring a rapid liquefaction of the slag-forming substances, and conse quently a high reactivity of the slag as well as a great capacity for absorbing oxidized constituents of the pig iron and for binding these products of oxidation. lf the oxygen jet impinges on the surface of the bath in a small area and with a heavy specific surface pressure, so that it penetrates into the bath, the zone of the highest tem perature does not develop on the surface of the bath, but below in the bath itself, the consequence being that the slag is not liquied so rapidly. lf on the other hand, the same quantity of oxygen hits the bath on a larger area. the reaction between the oxygen and the bath is not sufiiciently concentrated, the temperature in the reaction area being lower which has the same disadvantageous effect on the liqnefaction of the slag. Furthermore, by restricting the reaction to a reasonably defined zone on the surface of the steel bath, the carbon combustion is localized, causing already in an early stage a circulation in the bath, continually forwarding new parts of the bath to the center of reaction and thus speeding up refining. Localizing carbon combustion in the manner described has also the effect that numerous CO bubbles are formed in the molten metal beneath the center of reaction, thus reducing the specific weight of the bath in that area, which consequentlyr causes the liquid metal to rise at that point, and, having arrived at the surface, to move away from the reaction zone, whereas outside of said zone a descending current of molten metal develops, corresponding to the rising current at the reaction center.

The quantity of oxygen introduced per unit of time is adjusted by suitably selecting the diameter of the nozzle and the gas pressure in the conduit before the nozzle, and the area of the zone of reaction and the pressure of the oxygen jet on the bath is adjusted by regulating the gas pressure supplied to the nozzle anal the distance of the nozzle from the bath. ln general, a distance between the nozzle and the metal bath exceeding 20 inches is preferred, e. g. in case of the weight ol the metal charged amounting to approximately 30 tons, the distance will preferably be between 30 and 60 inches. With such distances, the diameter of the nozzle may be kept small enough to assure the necessary cooling effect` whilst on the other hand, the quantity of oxygen fed per unit of time will bc sufficient, without the jet penetrating into the metal bath. But in this first section of the process it is also possible, without adversely affecting the quality of the finished refined steel, to blow the oxygen onto the bath with a high impact pressure so that the jet penetrates into the metal bath or that, at least, the slag and the molten metal are intensively mixed by the mechanical effect of the jet, causing a whirl.

Soon after the introduction of oxygen is started, the oxidation of Si, P and Mn begins, and it takes only a short time for this process to attain a high speed. lf the basicity of the slag is sufficient, SiOz and P205 are firmly bound up with the siag. However, in this section of the process not only Si, P and Mn are oxidized, but also some iron oxidized from the bath so that the Fe() content of the slag will increase.

Due to the exothermic refining reaction, the temperature rises continuously which causes the combustion of carbon to begin. As the temperature continues to rise the carbon combustion becomes more and more intensive. rllhe moment at which the combustion of carbon reaches its maximum velocity is dependent on the conditions of the process and principally on the temperature in the converter.

Beginning from the moment at which the maximum decarburizing speed is attained, or soon after, the introduction of oxygen is held on a level below that of the oxygen quantity which is theoretically required for the combustion of the carbon, so that the final product is a steel low in oxygen. Thus it is possible to carry out the process in such a manner that the introduction of oxygen is reduced preferably at half-time, or at least before the process enters into its last quarter, whereby the quantity of the oxygen fed remains below, and preferably much below, the quantity of oxygen required for the carbon combustion.

Practical experience has taught that it is best to reduce the flow of oxygen'containing gas after 8-16 minutes of the blowing operation and preferably after about l2 minutes. Also, in the preferred method, the impact pressure of the jet on an area whose distance from the blowing nozzle corresponds to the distance between the bath surface and the nozzle, is kept below 0.75 kg./Cm.2, and preferably below 0.5 lig/cm?, so that the reaction be tween the gas jet and the iron bath is substantially restricted to the surface of the latter, avoiding an intensive mechanical mixing and whirling of bath and slag under the actions of the jet. For instance, the refining process may also be carried out so that during the period of oxygen deficiency, the difference between the oxygen actually introduced into the bath and the quantity theoretically necessary in that period for oxidizing the carbon, is between 10% and 50% and preferably 30%. Calculated on the basis of the oxygen quantity per ton of metal charged, the introduction of oxygen in the second period of the process may be kept at or reduced to below 2.0 Nin.302,1 and preferably below 1.7 Nrn.3O2 per minute and per ton of metal charged. At the end of the heat, i. e. during no more than 10% of its total duration. the introduction of oxygen may, however, be increased above the quantity of oxygen theoretically required for carbon combustion. This may be advantageous if it is desired to reduce the carbon contents of the iron bath to a very low level. If steel of less than 0.10% carbon is to be produced, it may be advisable to again reduce the introduction of oxygen during the last 10% of the blowing period to preferably less than one half. As soon as the steel analysis desired is attained, which can be judged from the appearance of the process, or from steel samples drawn, the liquid steel is poured into a ladle and is cast. According to the requirements, the deoxidizing and alloying additions are introduced either into the converter after the termination of blowing, or directly into the ladle.

As the oxygen required per unit of time in the period of the heaviest carbon combustion depends on the cornposition of the carbon-containing iron bath and on the temperatures prevailing, the most favorable operating conditions are determined in the following manner: With a selected type of pig iron, several refining operations are carried out under the same conditions of charging and processing, but the rate of oxygen introduction in the period of the heaviest carbon combustion is changed for each heat, and the total oxygen consumption is determined for each heat. Further, the theoretical oxygen requirements are calculated according to the rules of chemistry on the basis of the composition of the metal charged and the steel to be produced. Finally, the rela- 1 The symbol NniOs means cubic meters of oxygen at 0 C. and a pressure of one atmosphere gauge.

tion of the two oxygen values is ascertained. In such a manner, it has been determined that the heats in which the consumption of oxygen is less than 112%, and preferably about 105%, are those which best meet the conditions constituting the invention.

It has been found also that the operators have some leeway in the control of the process, for example, they may hold the introduction of oxygen per unit of time somewhat low within the range determined so as to obtain a steel of low oxygen content. A decreased rate of blowing, however, has the economical disadvantage of lengthening the blowing period. On the other hand, the oxygen may be blown at a higher rate per unit of time during the period of the heaviest carbon consumption within the range indicated which results in a somewhat higher total oxygen consumption and, of course, a somewhat higher oxygen content in the steel.

For a better understanding of the invention, reference may be had to the accompanying drawings in which:

Figs. 1 and 2 are graphs explaining the refining process according to Exam-ple 1; and

Fig. 3 is a diagram showing a device in operation for carrying out the process according to the invention.

The practice of the process according to the inven-tion is not restricted to the use of any particular plant or apparatus. Fig. 3 shows by way of example a device for practicing the process including an unheated converter or receptacle 1. The converter may have a door lining 2 and a wall lining 3 of suitable refractory brick or the like to receive the metal charge 5 having its surface 4 in contact with the slag 6. The gaseous refining means is blown at 7 through the lance 8 referred to above. The directions of the motions of circulation in the bath and in the slag are indicated by arrows. The waste gases escape through the mouth 9 of the converter and it is charged through the mouth 9.

The invention will be described with reference to typical examples of the method and with various types of charges for the converter. It will be understood that examples given hereinafter are illustrative and are not intended to limit the invention.

Example 1 After tapping, the converter l, shown in Fig. 3, is tilted for the introduction, by means of a movable inclined chute, of 3600 kg. of scrap, containing, on an average, 0.07% C, 0.40% Mn, 0.00% Si, 0.015% P and 0.025% S. Then, without altering the position of the converter 26,400 kg. of hot metal (4.16% C, 1.99% Mn, 0.12% Si, 0.070% P and 0.060% S) are introduced from a pig iron ladle. Then the converter is returned to its vertical position and is, by means of an inclined chute, fed with 1375 kg. incompletely burnt lime of 85% CaO, corresponding to 1170 kg. CaO, with 300 kg. mill scale of 62.28% FeO, and 28.4% FezOa, as well as with 530 kg. of crushed lire clay bricks, containing 65% SOz and AlzOs, corresponding to 345 kg. Si02.

As soon as the last-mentioned additions have been introduced, a lance 8, fitted with the oxygen nozzle, is run into the converter 1, from above and is lixed in such a position that the distance between the metal bath and the tip of the lance is about 48 inches. Then through the nozzle technically pure oxygen of 98% O2 is blown on the surface of the bath. The inner diameter of the nozzle is 1.2 inches. Blowing is carried on for 14 minutes with a pressure in the oxygen line of 8% kg./cn^1.2 above atmospheric. The pressure on the surface of the bath corresponds to a maximum specific surface pressure of the jet of oxygen of 0.60 kg./cni.2 on a surface whose distance from the blowing nozzle is equal to the distance of the bath surface from the blowing nozzle. At the end of 14 minutes of blowing, the pressure in the oxygen line is reduced to 6 lig/cm.2 above atmospheric, or equivalent to a maximum specific surface pressure of the jet of 0.42 lig/ern.2 on a surface whose distance from the blowing nozzle corresponds to the distance of the bath surface from the blowing nozzle. Under these operating conditions, blowing is continued for an additional 9 minutes, whereupon, after a total blowing period of 23 minutes, the lance is withdrawn from the converter and introduction of oxygen stops. Then the converter is tilted, a sample is drawn, and the finished blown steel is run into the ladle and cast unkilled. The analysis of the finished steel in the example given is as follows: 0.07% C, 0.39% Mn, 0.00% Si, 0.014% P, 0.023% nitrogen and 0.042% oxygen. The weight of the liquid steel in the finished state is found to be 27300 kg. corresponding to an output of 91%. The total amount of oxygen of 98% purity consumed was i377 Nm,1 corresponding to 1350 Nm.:l of pure oxygen or to a consumption of 45 Nn1.-`s of pure oxygen or 63.9 kg. per ton of metal charged.

With a pressure in the oxygen line of 8% kg./cm.2 above atmospheric the nozzle supplies 65.8 Nm. minute of oxygen of 98%, corresponding to 64.5 Nm.a or 91.5 kg. of pure oxygen per minute.

With a pressure in the oxygen line of 6 kg./cm."` above atmospheric the oxygen supply of the nozzle amounts to 50 Nm3/min., corresponding to 49 Nm3* or 69.6 kg. of pure oxygen.

Considering the compositions stated above, 1000 kg. of metal charged, consisting of 880 kg. pig iron and 120 kg. scrap contain 36.69 kg. C, 17.99 kg. Mn, 1.06 kg. Si, 0.64 kg. P and 0.56 kg. S. The 910 kg. of nished refined steel originating from the 1000 kg. of metal charged, contain 0.64 kg. C, 3.55 kg. Mn, 0.00 kg. Si, 0.13 kg. P and 0.21 kg. S. Hence 36.05 kg. C, 14.44 kg. Mn, 1.06 kg. Si, 0.51 kg. P and 0.35 kg. S have been oxidized in the course of the process. In accordance with the rules of stoichiometric chemistry, the following amounts of oxygen are necessary: For oxidation of l kg. C to C0 and 10% to CO2) 1.47 kg. oxygen; for oxidation of 1 kg. Mn to MnO-0.29 kg. oxygen; for oxidation of 1 kg. Si to SiOz1.14 kg. oxygen; for oxidation of 1 kg. P to Pros-1.29 kg. oxygen and for oxidation of 1 kg. S to SO2-1 kg. oxygen. Consequently, the consumption of oxygen per ton of metal charged during the entire process amounts to 52.9 kg. for the combustion of carbon, to 4.2 kg. for the oxidation of Mn, to 1.2 kg. for the oxidation of Si, to 0.66 kg. for the oxidation of P and to 0.35 kg. 02 for the oxidation of S, totalling 59.4 kg. On per ton of metal charged. For the oxidation of the 6.29 kg. Fe to FeO in the converter, which are contained in the l0 kg. dust-loss in the waste gases per ton of metal charged, 1.80 kg. Oz are consumed. Accordingly, the theoretical oxygen consumption for this process amounts to 59.4 and 1.80 kg. i. e. 61.2 kg. O2 per ton of metal charged. As explained above, the pure oxygen consumption for blowing has been ascertained at 63.9 kg. O2 per ton of metal charged. In addition, 2.2 kg. 02 are available out of the iron oxides contained in the 10 kg. rolling mill sinter, added per ton of metal charged, so that the real total consumption of oxygen in that heat amounted to 66.1 kg. per ton of metal charged, or 108% of the theoretical oxygen requirements.

As could be ascertained by comparing the analysis of the samples drawn out of the converter during the blowing phase, the combustion of carbon reached its maximurn value in the twelfth minute of blowing (0.24% carbon combustion per min.). It was at 0.24% C/min. between the fourteenth and the sixteenth minutes, at 0.22% C/min. between the sixteenth and the eighteenth minutes, at 0.21% C/min. between the eighteenth and the twentieth minutes and at 0.18% C/min. between the twentieth and the twenty-second minutes. If the weight of the liquid iron is calculated for the various periods mentioned above, taking into account the combustion of the accompanying elements and the other losses, the following values are obtained. In the period between the fourteenth 1The symbol Nm.3 means cubic meters at 0 C. and one atmosphere pressure gauge.

and the sixteenth minutes, i. e. for the first two minutes after the reduction of the oxygen quantity, the weight of the iron bath amounts to 934 kg. per ton of metal charged. A combustion of 0.24% C out of that iron bath corresponds to a combustion of 2.24 kg. C/min. and ton of metal charged, or of 67.2 kg. C/min. calculated on the entire weight of the metal charged. For the combustion of 90% of that carbon amount to CO and of 10% to CO2, 98.7 kg. pure oxygen per min. are required. As the quantity of oxygen, fed under a pressure of 6 lig/cm.2 above atmospheric in the oxygen line, amounted to 69.6 kg. pure oxygen/min., as stated above, the gaseous O2 introduced in the period between the fourteenth and the sixteenth minutes was only 70.5% of the O2 requirements for the carbon combustion arising in the same period.

The same method of calculation reveals for the time between the twentieth and twenty-second minutes a weight of the iron bath of 919 kg. per ton of metal charged corresponding to a carbon combustion of 0.18 C/min., and consequently, to 1.66 kg. C of burnt carbon per minute and per ton of metal charged. Hence, considering the entire weight of the metal charged, 49.8 kg. C/min. have been burnt between the twentieth and the twenty-second minutes, while 73.4 kg. Oz/min. were necessary. As the oxygen quantity fed amounted to 69.9 kg. Oz/min., as indicated above, it is manifest that even in the two minutes, i. e. from the twentieth to the twentysecond, or in other terms, one minute before the termination of the process, the quantity of gaseous oxygen actually introduced was only 95% of the quantity theoretically required for carbon combustion.

For facilitating the understanding of the reactions de scribed with reference to the above example, a graph (Fig. l) has been evolved showing the development in time of the refining process, the times in minutes being plotted on the abscissa, and the percentual contents of the bath in iron accompanying elements Si, P, S, C, Mn being plotted on the ordinate.

Fig. 2 discloses the curve (A) of the oxygen introduction per minute for the entire metal charge of 30 tons to which Fig. 1 refers, as well as the oxygen requirements/min. (B) for the carbon combustion between the eighth and the tenth minutes, likewise with reference to the total weight charged. The graph shows at a glance that, beginning from the period between the tenth and the twelfth minutes, the introduction of oxygen/minute is inferior to the oxygen requirements for carbon combustion, and that nonetheless the introduction is, shortly after, further reduced so that it remains below the theoretical oxygen quantity for carbon combustion up to the last of the duration of the heat, the ordinates showing the pure oxygen in kg./min. and the abscissa the duration of the process.

Example 2 After the termination of the preceding heat, the converter was tapped and tilted for introducing 4500 kg. of scrap, having an average composition of 0.07% C, 0.40% Mn, 0.00% Si, 0.015% P, 0.025% S. Then, without changing the position of the converter, 22,500 kg. of hot pig iron containing 4.20% C, 2.20% Mn, 0.18% Si, 0.070% P and 0.050% S were introduced into it from a mixing ladle, whereupon the converter was turned back to its vertical position and was fed by means of an inclined chute, with 1400 kg. of incompletely burnt lime, containing 85% CaO and corresponding to 1190 kg. CaO. with 300 kg. of mill scale of 62.28% FeO and 28.4% FezOs and with 500 kg. crushed iireclay bricks, containing 65% Si02, and 20% A1203 which corresponds to 325 kg. SiOz.

After the last-mentioned additions had been introduced, the nozzle pipe carrying the oxygen nozzle was immediately run in from above and was so located that the distance of the nozzle mouth from the metal bath amounted to 1200 mm. (48 inches). Then technically pure oxygen containing 98% Oz was blown on the surface of the bath. The inner diameter of the nozzle was 30 mm. (1.2 inches). During the first 14 minutes, blowing was effected at a pressure of 10 Lrg/ern.2 above atmospheric in the oxygen line, corresponding to the maximum specific surface pressure of the blowing jet of 0.70 kg./cm.2, as exerted on a surface whose distance from the blowing nozzle corresponds to the distance of the bath surface from the nozzle.

Then the pressure in the oxygen introduction line was reduced to 5 kg./cm.2 above atmospheric corresponding to a peak specific surface pressure of 0.35 kg./cm.2 on a surface whose distance from the blowing nozzle corresponds to the distance between the bath surface and the blowing nozzle.

Blowing is continued under these conditions for 6 minutes, whereupon, after a total blowing period of 20 minutes, the nozzle is withdrawn from the converter and the introduction of oxygen stopped. The exterior appear ance and the characteristic features of the process observed indicated at that time that the desired carbon content of the steel had been attained. Then the converter was tilted, samples were drawn and the finished blown steel was poured into a ladle and cast unkilled. The analysis of the finished blown steel was the following: 0.08% C, 0.40% Mn, 0.00% Si, 0.018% P, 0.025% S, 0.003% N2 and 0.032% O2.

The weight of the finished blown liquid steel was found to be 27,300 kg. corresponding to an output of liquid raw steel of 91%. The total consumption of oxygen of 98% amounted to 1300 Nm, equal to 1274 Nm.3 of pure oxygen. Hence, the consumption of pure oxygen was 42.5 Nm3 or 60.4 kg. per ton of metal charged. At a pressure in the oxygen line of 10 kg./cm.2 above atmospheric, the nozzle supplied Nm.3/min. of oxygen of 98%, corresponding to 73.5 Nm3/min. or 104.4 kg. of pure oxygen per minute. With a pressure in the oxygen line of 5 kg./cm.2 above atmospheric, the nozzle supplied 41.6 Nm3/min. corresponding to 40.7 Nm3 or 57.8 kg. of pure oxygen/ min.

With the composition mentioned above, 1000 kg. of metal charged comprising 850 kg. of pig iron and kg. scrap contain 35.8 kg. C, 19.30 kg. Mn, 1.53 kg. Si, 0.62 kg. P, 0.46 kg. S. The 910 kg. of finished blown refined liquid steel, produced from 1000 kg. of metal charged, contain 0.73 kg. C, 3.64 kg. Mn, 0.00 kg. Si, 0.16 kg. P, 0.23 kg. S. Consequently, the following quantities have been removed in the course of the process: 35.07 kg. C, 15.66 kg. Mn, 1.53 kg. Si, 0.45 kg. P, 0.23 kg. S. According to the rules of stoichiometric chemistry, the amount of oxygen necessary for oxidation of 1 kg. C (90% to CO and 10% to CO2) is 1.47 kg. oxygen; for oxidation of 1 kg. Mn to MnO-0.29 kg. oxygen; for oxidation of 1 kg. Si to SiO2-1.14 kg. oxygen; for oxidation of l kg. P to P2O5-l.29 kg. oxygen, and for oxidation of 1 kg. S to SO2-1 kg. oxygen.

The oxygen requirements per ton of metal charged during the entire process amount to: For the combustion of carbon-51-55 kg.; for the oxidation of Mn-4.54 kg.; for the oxidation of Si-1.74 kg.; for the oxidation of P-0.58 kg.; and for the oxidation of S-0.23 kg. O2, totalling 58.65 kg. O2 per ton of metal charged. For the oxidation in the converter of the 6.29 kg. Fe to FeO contained in the 10 kg. of dust lost, escaping in the waste gases, 1.80 kg. O2 are consumed. Thus the theoretical oxygen requirements for carrying out the process amount to 58.65 and 1.8 kg., i. e. 60.45 kg. Oz per ton of metal charged. As explained above, the pure oxygen consumption, out of the 98% oxygen employed for blowing, has been ascertained at 60.4 kg. O2 per ton of metal charged. Furthermore, 2.2 kg. Oa were available, taken from the iron oxides of the 10 kg. rolling mill sinter added per ton, so that the total oxygen consumption in this heat amounted to 62.6 kg. per ton of metal charged or 103.4% of the theoretical oxygen requirements.

As the analysis of the samples, drawn from the converter during the blowing process showed, the carbon combustion reached its maximum value in the twelfth minute with a carbon combustion/min. of 0.27%. Between the fourteenth and the sixteenth minutes the carbon combustion was 0.20% C/min., between the sixteenth and the eighteenth minutes 0.15% C/min., and between the eighteenth and the twentieth minutes 0.15% C/min. Posed on the reduction of the weight resulting from combustion and other loses, it is possible to calculate the weights corresponding to the different periods of the process as stated above. For the period between the fourteenth and the sixteenth minutes, i. e. for the first two minutes after the reduction of the oxygen, the weight of the iron bath was found to be 933 kg. per ton of metal charged. A combustion of 0.20% C out of that iron bath corresponded to a combustion of 1.86 kg. C/min. and ton of metal charged or 55.8 kg. C/ min., calculated on the basis of the total weight of metal charged. For the combustion of 90% of that carbon quantity to C0 and of 10% to CO2, 82 kg. of pure oxygen are required per minute. In view of the fact that the oxygen introduction under a pressure of kg./cm.' above atmospheric in the oxygen line, as stated above, amounted to 57.8 kg. of pure oxygen, the quantity of gaseous O2 fed between the fourteenth and the sixteenth minutes was but of 70.5% of the oxygen requirements for the carbon combustion arising in the same time.

A calculation of the same kind, based on the period between the eighteenth and the twentieth minutes, yields the following result: The weight of the iron bath is 920 kg. per ton of metal charged and the carbon combustion amounts to 0.15% C/minute which corresponds to a burnt quantity of carbon of 1.38 kg. C/min. and per ton of metal charged. With reference to the total weight of metal charged, between the eighteenth and the twentieth minutes 41.4 kg. C/rnin. have been burned for which purpose 60.8 kg. Oz/min. were necessary. As the utilization of oxygen rose but to 57.8 kg. Ozfmin., the quantity of oxygen fed between the eighteenth and the twentieth minutes, i. e. shortly before the end of the process was still at 95%- below the oxygen quantity necessary for the combustion of the carbon.

Example 3 After the termination of the preceding heat the converter was tilted and 3900 kg. of scrap were charged having an average composition as follows: 0.07% C., 0.40% Mn, 0.00% Si, 0.015% P, 0.025% S. Then without altering the position of the converter 26,100 kg. of hot metal, having a composition of 4.00% C, 2.40% Mn, 0.15% Si, 0.090% P and 0.050% S were charged into the converter from a mixing ladle. Then the converter was returned to its vertical position and the following materials were charged: 1600 kg. of incompletely burnt lime containing 85% CaO and corresponding to 1360 kg. CaO; 500 kg. of mill scale containing 62.28% FeO and 28.4% FesOs and 400 kg. crushed tire clay bricks containing 65% SiOz and 20% A1203, corresponding to 260 kg. SiO2. After the introduction of the abovementioned additions, the lance is immediately run from above Yinto the converter and is positioned so that the space between the nozzle outlet and the metal bath is of 1400 mm. (56 inches). Then blowing is started, a jet of technically pure oxygen of 98% O2 being directed onto the surface of the bath. The interior diameter of the nozzle was 30 mm. (1.2 inches). During the first 14 minutes, blowing is carried on at a pressure in the oxygen line of kg./cm.2 above atmospheric, corresponding to a peak specific surface pressure of the blowing jet of 0.47 kg./cm.2 on an area whose distance from the blowing nozzle is equal to the distance of the bath surface from said nozzle.

Then the oxygen line pressure was reduced to 6 lig/cru.2

above atmospheric corresponding to a peak specie surface pressure of the blowing jet of 0.28 lag/cm.:x on an area located at a distance from the blowing nozzle equal to the distance of the bath surface fromthe blowing nozzle. Under these conditions blowing was continued for another 6 minutes, whereupon i. e. after a total blowing time of 20 minutes, the nozzle was withdrawn from the converter, and the introduction of oxygen stopped. The outward appearance and the characteristic features of the process observed indicated at this time that the carbon content of the steel had been reduced to the desired value. Thereon the converter was tilted, and after the samples had been drawn the finished blown steel was poured into the ladle from which it was cast unkilled.

The analysis of the finished blown steel was the following: 0.06% C, 0.36% Mn, 0.00% Si, 0.014% P, 0.022% S, 0.002% N2 and 0.038% O2. The weight of the nished blown liquid steel was found to be 27,300 kg. corresponding to an output of 91% in liquid raw steel. The total consumption of 98% oxygen amounted to 1350 Nm3, corresponding to 1323 Nrn.3 of pure oxygen, so that the consumption in pure oxygen works out at 44.1 Nm.a or 62.6 kg. per ton of metal charged. At a pressure in the oxygen line of 10 lig/cm? above atmospheric the nozzle suppiies 75 Nm3/min. of 98% oxygen, corresponding to 73.5 Nina/min. or 104.4 kg. pure oxygen/ min. At a pressure in the oxygen line of 6 lig/cm.3 above atmospheric the supply of oxygen by the nozzle amounted to 50 Nm3/min., equal to 49 Nrn.3 or 69.6 kg. of pure oxygen per minute.

On the basis of the above compositions 1000 kg. of metal charged, consisting of 870 kg. pig iron and kg. scrap contain 3489 kg. C, 21.40 kg. Mn, 1.30 kg. Si, 0.80 kg. P and 0.47kg. S. On the other hand, the 910 kg. of finished blown liquid steel, originating from 1000 kg. of metal charged, contains: 0.55 kg. C, 3.28 kg. Mn, 0.00 kg. Si, 0.13 kg. P and 0.20 kg. S. Consequently, 34.34 kg. C, 18.12 kg. Mn, 1.30 kg. Si, 0.67 kg. P and 027 kg. S have been oxidized in the course of the process. According to the rules of stoichiometric chemistry, the following amounts of oxygen are necessary: For oxidation of 1 kg. C (90% to CO and 10% to CO2), 1.47 kg. oxygen; for oxidation of 1 kg. Mn to MuO, 0.29 kg. oxygen; for oxidation of 1 kg. Si to SiOz, 1.14 kg. oxygen; for oxidation of 1 kg. P to P205, 1.29 kg. oxygen, and for oxidation of 1 kg. S to SO2, 1 kg. oxygen.

Hence, the consumption of oxygen (O2) per ton of metal charged during the whole process amounts to 50.48 kg. for the combustion of carbon; 5.54 kg. for the oxidation of Mn; 1.48 kg. for the oxidation of Si; 0.86 kg. for the oxidation of P, and 0.27 kg. for the oxidation of S. This makes 58.63 kg. O2 per ton of metal charged. For the oxidation in the converter of the 6.29 kg. Fe to FeO contained in the 10 kg. of dust per ton of metal charged, which dust is lost by way of the waste gases, 1.80 kg. Oz are consumed. All in all, the theoretical oxygen requirements for this process amount to 58.63 plus 1.80 kg. i. e. 60.48 kg. O2 per ton of metal charged. As ascertained by way of measurements, the pure oxygen consumption out of the 98% oxygen used for blowing was, as explained above, 62.6 kg. O2 per ton of metal charged. In addition, 3.7 kg. O2 were available out of the iron oxides of the 16.7 kg. rolling mill sinter per ton of metal charged which had also been added, so that the total oxygen consumption in that heat amounted to 66.3 kg. per ton of metal charged or 109.7% of the theoretical oxygen requirements.

As was disclosed by the analysis of the samples, drawn from the converter during the blowing process, the carbon combustion in the twelfth minute reached its peak value at 0.27% carbon combustion/minute. Between the fourteenth and the sixteenth minute the combustion was at the level of 0.20% C/min., between the sixteenth and the eighteenth at the level of 0.21% C/min., and between the eighteenth and the twentieth minute at the level of 0.20% C/min. Calculating the weights of the liquid iron for the above-mentioned periods and taking therein into account the weight reductions caused by the removal of the accompanying elements and the other losses, the following result is attained: During the period from the fourteenth to the sixteenth minute, i. e. for the rst two blowing minutes after the reduction of the oxygen quantity, the weight of the iron bath has been found to be 934 kg. per ton of metal charged. The combustion of 0.20% C out of that bath corresponds to a removal of 1.86 kg. C/minute and ton of metal charged, or to the removal of 55.8 kg. C/min., with reference to the total weight charged. For the combustion of 90% of that carbon quantity to CO and of 10% to CO2, 82.0 kg. pure oxygen/ min. are required. As with a pressure of 6 kg./cm.2 above atmospheric in the oxygen line, the introduction of pure oxygen, as stated above, amounted to 69.6 kg. of pure oxygen/minute the quantity of gaseous oxygen actually supplied in the period from the fourteenth to the sixtcenth minutes was only 84.8% of the Oz requirements for carbon combustion arising at the same time.

The same calculation yields, for the period from the eighteenth to the twentieth minute, a weight of the iron bath of 921 kg. per ton metal charged, corresponding to a carbon combustion of 0.20% C/rnin., i. e. to a burnt quantity of carbon of 1.84 kg. C/min. and ton of metal charged. Hence, calculated on the basis of the entire quantity of metal charged, between the eighteenth and the twentieth minute 55.2 kg. C/min. have been burnt, for which purpose 81.1 kg. Oz/min. were necessary. As stated above, the introduction of oxygen amounted to 69.6 kg. Oz/min. so that the actual quantity of gaseous oxygen fed from the eighteenth to the twentieth minute, i. e. shortly before the end of the process, was at 85.8%, or in other terms, was below the quantity of oxygen required for the carbon combustion.

From the preceding examples, it will be clear that a process is provided whereby steel comparable to open hearth steel can be produced by means of a surface blowing operation with close to theoretical consumption of oxygen and in such a short period of time and with such simple equipment as to render it competitive with open hearth practice.

It will be understood that the process is capable of variation in the blowing rate and conditions and in the raw metal undergoing rening and, accordingly, the examples set forth above should be considered as illustrative, only, of the method.

We claim:

l. A process for manufacturing steel of low oxygen content comprising blowing substantially pure oxygen from above against the surface of a bath of molten crude iron containing impurities including carbon, sulfur and silicon and having a Huid slag cover in a converter type vessel at a rate high enough to oxidize sulfur, silicon and other impurities in said metal and provide an amount of oxygen in excess of the theoretical oxygen requirement per unit of time based on carbon combustion rate, continuing blowing at said rate until the bath of molten iron has reached a temperature between about 1450 and 1600 C., then reducing the flow of oxygen against the surface of the bath to a lower rate not exceeding 2.0 normal cubic meters per minute per ton of metal, and continuing blowing the bath with oxygen at said lower rate until at least about 90% of the blowing time has lill 416 elapsed, and discontinuing blowing when not more than 112% of the theoretical volume of oxygen required to oxidize said impurities to the degree desired in the finished steel has been blown into said vessel.

2. The process set forth in claim 1 in which the blowing rate is reduced to less than said lower rate during about the last 10% of the blowing time.

3. A process for manufacturing steel of low oxygen content comprising blowing substantially pure oxygen from above against the surface of a bath of molten crude iron containing impurities including carbon, sulfur and silcon and having a uid slag cover in a converter type vessel at a rate high enough to oxidize sulfur, silicon and other impurities in said metal and provide an amount Vof oxygen in excess of the theoretical oxygen requirement per unit of time based on carbon combustion rate, continuing blowing at said rate until the carbon content of the molten iron has decreased to about 75% to 25% of the original carbon content of the molten iron, then reducing the tiow of oxygen against the surface of the bath to a lower rate not exceeding 2.0 normal cubic meters per minute per ton of metal, and continuing blowing the bath with oxygen at said lower rate until at least about 90% of the blowing time has elapsed, and discontinuing blowing when not more than 112% of the theoretical volume of oxygen required to oxidize said impurities to the degree desired in the finished steel has been blown into said vessel.

4. The process set forth in claim 3 in which the blowing rate is reduced to a lower rate not exceeding 2.0 normal cubic meters per minute per ton of metal when the carbon content of the molten iron has decreased to about 50% of the original carbon content of the molten iron.

5. The process set forth in claim 3 in which blowing the bath with oxygen is discontinued when not more than 105% of the theoretical volume of oxygen required to oxidize said impurities to the degree desired in the finished steel has been blown into said vessel.

6. A process for manufacturing steel of low oxygen content comprising blowing substantially pure oxygen from above against the surface of molten predominately pig iron containing metal having a uid slag cover in a converter type vessel at a rate substantially higher than 2.0 normal cubic meters per minute per ton of metal in said vessel, to oxidize sulfur, silicon and other impurities in said metal and provide an amount of oxygen in excess of the theoretical oxygen requirement per unit of time based on carbon combustion rate, continuing blowing at said rate until the molten metal has reached a temperature between about 1450 C. and 1600 C., then reducing the fiow of oxygen against the surface of the molten metal to a lower rate not exceeding 2.0 normal cubic meters per ton of metal, and continuing blowing the molten metal with oxygen at said lower rate until not more than 112% of the theoretical volume of oxygen required to oxidize said impurities to the degree desired in the finished steel has been blown into said vessel.

References Cited in the tile of this patent FOREIGN PATENTS 509,998 Belgium Apr. 15, 1952 

1. A PROCESS FOR MANUFACTURING STEEL OF LOW OXYGEN CONTENT COMPRISING BLOWING SUBSTANTIALLY PURE OXYGEN FROM ABOVE AGAINST THE SURFACE OF A BATH OF MOLTEN CRUDE IRON CONTAINING IMPURITIES INCLUDING CARBON, SULFUR AND SILICON AND HAVING A FLUID SLAG COVER IN A CONVERTER TYPE VESSEL AT A RATE HIGH ENOUGH TO OXIDIZE SULFUR, SILICON AND OTHER IMPURITIES IN SAID METAL AND PROVIDE AN AMOUNT OF OXYGEN IN EXCESS OF THE THEORETICAL OXYGEN REQUIREMENT PER UNIT OF TIME BASED ON CARBON COMBUSTION RATE, CONTINUING BLOWING AT SAID RATE UNTIL THE BATH OF MOLTEN IRON HAS REACHED A TEMPERATURE BETWEEN ABOUT 1450* AND 1600*C., THEN REDUCING THE FLOW OF OXYGEN AGAINST THE SURFACE OF THE BATH TO A LOWER RATE NOT EXCEEDING 2.0 NORMAL CUBIC METERS PER MINUTE PER TON OF A METAL, AND CONTINUING BLOWING THE BATH WITH OXYGEN AT SAID LOWER RATE UNTIL AT LEAST ABOUT 90% OF THE BLOWING TIME HAS ELAPSED, AND DISCONTINUING BLOWING WHEN NOT MORE THAN 112% OF THE THEORETICAL VOLUME OF OXYGEN REQUIRED TO OXIDIZE SAID IMPURITIES TO THE DEGREE DESIRED IN THE FINISHED STEEL HAS BEEN BLOWN INTO SAID VESSEL. 